This invention relates to electromechanically actuated impact devices for use in high speed printers, punches or the like; more particularly to a modular construction of magnetically driven impact devices and to improvements in actuator construction and in the arrangement of magnets and electrical actuating means for driving impact devices.
Although in its broader aspects the invention is not limited thereto, the various features of the invention have particular applicability to high speed line printers of the impact type.
As is well known in the art, high speed line printers of the impact type typically employ a series of print hammers disposed along a print line in side-by-side relationship, there being one print hammer for each location at which information is to be printed on a line. In order to provide a readable format, the column or print spacing along a print line in a typical computer print-out should be about 1/10 of an inch or even less. A number of problems arise in the design of hammers which are close enough together to meet this spacing limitation and yet are capable of operating reliably over prolonged periods of time at the high speeds demanded by the data processing industry.
In accordance with one body of prior art, print hammers of the kind referred to are individually actuated by means of solenoids which impart movement to the hammers through linkages or mechanically moveable striker members. Such arrangements are limited to use in relatively slow speed applications due to lost motion in the mechanical connections as well as the losses due to friction and inertia. In addition, in high speed applications, the solenoids must be relatively large and when an assembly of solenoid and connecting linkages is taken into account, complex spacing, alignment, assembly and servicing problems arise. Relatively large and expensive power supplies are required for operation of the solenoids. Moreover, at the speeds required of printers of the type to which a preferred embodiment of the invention relates, overheating of the solenoids becomes a significant problem, requiring that means for ventilating and cooling be provided.
Significant advances in impact printers are disclosed in Irwin et al U.S. Pat. No. 3,087,421 and Kalbach et al. U.S. Pat. No. 3,282,203. According to these patents, printing speeds of over a thousand lines per minute are achieved by the use of hammers carrying coils which are located in gaps formed between a series of spaced apart permanent magnets, with the faces of the magnets forming the gaps being of opposite polarity so that a field extends across each gap. According to these patents, one gap is provided for each hammer and the gaps extend in the direction of movement of the hammers so that current impulses through the coils cause movement of the hammers against an impact surface.
In order to maintain a print spacing of at least 10 characters per inch, the prior art described just above makes use of a plurality of rather small, thin flat magnets, which are separated by narrow gaps. The magnets are staggered above and below the print line in order to maintain the gaps on the 1/10 of an inch centers dictated by a 10 character per inch print spacing and are usually arranged so that adjacent hammers are suspended on supports which are alternately located above and below the magnets in order to provide clearance between the supports and magnets. Obviously, a drawback to the approach shown in these patents is that it requires a very large number of magnets which must closely and accurately spaced in side-by-side relationship. Flat coils must be carefully made in order to fit within the narrow spaces. Perfect alignment of the parts is needed in order to limit frictional contact of the flat coil with the adjacent magnet faces. Since the coils must be extremely thin, the number of turns and the size of wire on each coil is limited. Typically, thin aluminum foil is used for the coil wire. To compensate for the limited number of turns, the magnets used have a relatively high flux density and a relatively high current is passed through the aluminum foil material to provide the needed driving force. Because of the high current values, the coils which are confined in the narrow gaps tend to heat up. Hysterisis losses and eddy currents cause further heating. In addition, the eddy currents delay the build-up of the magnetic field. A relatively large power supply must be used to compensate for the thin coils and this disproportionately increases the cost of the equipment.